The wheels of a skateboard are typically made of polyurethane and come in many different sizes and shapes adapted to different types of skating.
It is also well known that the properties of the surface material affect the behavior of the skateboard. Polyurethane, which can be found with different friction coefficients, rolling resistance, and rebound depending on the mechanical properties of the material, such as its hardness. Hardness is usually measured on a Shore durometer scale in the range of Shore A 75 to Shore A 100 or harder. For example, hard wheels can slide more easily while softer wheels are can maintain higher speeds without sliding. Skateboard wheels have a wide surface engaging the ground, ranging from approximately 1 cm to over 5 cm. Traditionally, skateboarders have had to make compromises between control and a smooth ride on one hand and high speed on the other. Thinner wheels are generally made of relatively hard urethane, facilitating slides, grinds and other tricks. Thicker wheels are typically made with softer urethane for more control, making them suitable for, for example, downhill racing.
U.S. Pat. No. 6,953,225 discloses a skateboard wheel as initially defined. This wheel has a radial surface comprising outer portions and an inner portion between the outer portions, with a substantially linear border between them. Each of the three portions extends completely around the circumference of the wheel. The outer portions are made from a harder material than the inner portion, giving the outer portions a lower friction and thereby properties suitable for making tricks, especially involving sliding, while the inner portion has a higher friction thereby providing a higher degree of control, according to U.S. Pat. No. 6,953,225. The wheel has an axle passage through the centre with a bearing surface facing a hub for mounting the wheel on a skateboard. Hence, a designer of skateboard wheels can vary the properties of the wheels through selection of the hardness and friction coefficient of the materials used on the surface, and the width of the outer and inner portions of the wheels Similar considerations apply to other types of wheels, for example for roller skates or inline roller skates.
U.S. Pat. No. 4,699,432 discloses a safety wheel for use with, for example, roller skates or skateboards. The wheel is designed to provide improved traction and performance and comprises portions of a first material having a relatively low friction coefficient and a second material, softer than the first material and having a higher friction coefficient.